The tensor-product conjecture for torsion subalgebras of integral-dynamics Kirchberg algebras

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Let S⊂N×∖{1}S\subset\mathbb{N}^{\times}\setminus\{1\} be a family of relatively prime numbers with ∣S∣≥2\lvert S\rvert\geq 2. Let AS\mathcal{A}_S be the torsion subalgebra associated to SS, let QS\mathcal{Q}_S be the corresponding unital UCT Kirchberg algebra, and let gSg_S denote the greatest common divisor of {p−1:p∈S}\{p-1:p\in S\}. For an index set of the appropriate cardinality, write e1=(δ1,j)j∈(Z/gSZ)2∣S∣−2e_1=(\delta_{1,j})_j\in(\mathbb{Z}/g_S\mathbb{Z})^{2^{\lvert S\rvert-2}}. Tensor-product conjecture. The algebra AS\mathcal{A}_S is isomorphic to

⨂p∈SOp.\bigotimes_{p\in S}\mathcal{O}_p.

Equivalently, QS\mathcal{Q}_S is the unital UCT Kirchberg algebra with

(K0(QS),[1],K1(QS))=(Z2∣S∣−1⊕(Z/gSZ)2∣S∣−2,(0,e1),Z2∣S∣−1⊕(Z/gSZ)2∣S∣−2).(K_0(\mathcal{Q}_S),[1],K_1(\mathcal{Q}_S))=\left(\mathbb{Z}^{2^{\lvert S\rvert-1}}\oplus(\mathbb{Z}/g_S\mathbb{Z})^{2^{\lvert S\rvert-2}},(0,e_1),\mathbb{Z}^{2^{\lvert S\rvert-1}}\oplus(\mathbb{Z}/g_S\mathbb{Z})^{2^{\lvert S\rvert-2}}\right).

In particular, for non-empty sets S,T⊂N×∖{1}S,T\subset\mathbb{N}^{\times}\setminus\{1\} of relatively prime numbers, QS\mathcal{Q}_S is isomorphic to QT\mathcal{Q}_T if and only if ∣S∣=∣T∣\lvert S\rvert=\lvert T\rvert and gS=gTg_S=g_T. The conjecture extends the cases already established for ∣S∣≤2\lvert S\rvert\leq 2 and gS=1g_S=1; the general case with ∣S∣≥3\lvert S\rvert\geq 3 and gS>1g_S>1 is left open.

References

Primary source

Selçuk Barlak, Tron Omland and Nicolai Stammeier, “On the K-theory of C*-algebras arising from integral dynamics”, arXiv:1512.04496 (2016).

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